1. Field of the Invention
The present invention relates to batteries including rolled electrodes and particularly relates to a battery, such as a lithium ion polymer secondary battery, including rolled electrodes. The rolled electrode-including battery has a structure in which electrolyte layers, a cathode, and an anode are layered or rolled.
2. Description of the Related Art
In recent years, mobile electronic devices such as notebook personal computers, mobile phones, and personal digital assistants (PDAs) have been widely used. Therefore, lithium ion batteries are widely used as power supplies for such devices because the batteries have high voltage and energy density and is light in weight.
Since batteries containing electrolyte solutions have a problem in that the solutions leak out the batteries, a lithium ion polymer secondary battery including the following component has been commercialized: a solid electrolyte or a gel polymer membrane impregnated with a nonaqueous electrolyte solution.
The lithium ion polymer secondary battery has a cell structure in which a unit cell including a cathode, an anode, and a polymer electrolyte is covered with a jacket such as an aluminum laminate, the cathode and the anode each being connected to corresponding leads. The unit cell and a wiring board having a circuit section mounted thereon are housed in a plastic molded case consisting of an upper part and a lower part.
In order to reduce the internal resistance, the lithium ion polymer secondary battery must have a large reaction area. Therefore, the lithium ion polymer secondary battery is prepared by spirally rolling a plurality of electrodes depending on the thickness of the battery, whereby the battery is allowed to have an increased reaction area. A battery including a plurality of electrodes spirally rolled is herein referred to as a rolled electrode-including battery.
FIG. 4 is an illustration showing a lithium ion polymer secondary battery which is an example of a known rolled electrode-including battery and to which the present invention is applicable. The lithium ion polymer secondary battery includes a cathode, an anode, and an electrolyte section placed therebetween, these components being rolled to form a layered structure. The cathode includes a cathode current collector 1 and a cathode active material layer 2 attached thereto. The anode includes an anode current collector 7 and an anode mix layer 6 attached thereto.
The electrolyte section includes a cathode electrolyte layer 3 and an anode electrolyte layer 5 that are separated from each other by a separator 4. The battery further includes a cathode lead terminal 8, an anode lead terminal 9, and covers (protective tapes) 10a, 10b, 10c, 10d, 10e, 10f, and 10g and those components form a principal internal structure of the battery.
FIG. 5 is an enlarged view showing a layered portion of the lithium ion polymer secondary battery shown in FIG. 4. Reference numeral 11 represents the cathode, reference numeral 12 represents the anode, and reference numeral 13 represents the electrolyte section. The cathode current collector 1 includes a metal strip containing aluminum (Al) or the like. The cathode active material layer 2 includes a film containing an active material such as lithium (Li), cobalt (Co), manganese (Mg), nickel (Ni), cadmium (Cd), or graphite and a binder for bonding the cathode active material layer 2 to the cathode current collector 1.
The anode current collector 7 includes a metal strip containing copper (Cu). The anode mix layer 6 includes a film containing an active material such as a carbonaceous material or the like and a binder for bonding the anode mix layer 6 to the anode current collector 7.
The cathode electrolyte layer 3 and anode electrolyte layer 5 included in the electrolyte section 13 contain a gel polymer having good electrochemical properties. The gel polymer is bendable and flexible and contains an electrolyte prevented from leaking out. In order to satisfy such requirements, the electrolyte is preferably distributed in the gel polymer uniformly.
The separator 4 contains a material which can securely separate the cathode electrolyte layer 3 from the anode electrolyte layer 5 and which allows ions to substantially freely migrate between the cathode 11 and the anode 12. Examples of such a material include microporous polypropylene.
The covers 10a, 10b, 10c, 10d, 10e, 10f, and 10g are used to prevent the cathode 11 and the anode 12 from making contact with each other due to deterioration with age or the pressure applied to the battery from outside.
Japanese Unexamined Patent Application Publication No. 2001-266946 discloses a lithium ion battery including a cathode, an anode, an electrolyte layer, and an insulating cover. At least one end of one of the cathode and the anode is exposed from the electrolyte layer and the other one faces the exposed end and is covered with the insulating cover.
With reference to FIG. 6, a cathode lead electrode and anode lead electrode are connected to a plurality of electrodes spirally rolled and a unit cell 41 with a flat shape is covered with a jacket 31 laminated with aluminum. The jacket 31 has a recessed section 32 formed by a punching process and a terrace section 33. The jacket 31 further has an untreated folding section, not shown, for covering the recessed section 32. The unit cell 41 is placed in the recessed section 32, which is covered with the folding section, of which end portions are fixed to the terrace section 33 by thermal fusion.
In the known battery, a cathode lead terminal 8 is fixed to a leading portion of a rolled cathode current collector 1, the leading portion being not covered with a cathode active material layer 2, and an anode lead terminal 9 is fixed to a leading portion of an rolled anode current collector 7, the leading portion being not covered with an anode active material layer 3. Alternatively, the anode lead terminal 9 is fixed to a tail portion of the anode current collector 7 in some cases, the tail portion being not covered with the anode active material layer 3. In both cases, the cathode lead terminal 8 extends outward from the center of the coil of the rolled electrodes.
The recessed section 32 of the jacket 31 in which the unit cell 41 is placed is hermetically sealed. Therefore, in order to allow the cathode and anode lead terminals 8 and 9 to extend out of the recessed section 32, the cathode and anode lead terminals 8 and 9 must be bent into an L shape at their portions near a side face 42 of the unit cell 41 as shown in FIGS. 6 and 7, the side face 42 having portions from which the cathode and anode lead terminals 8 and 9 extend. Reference numeral 43 represents an outward face of the unit cell 41 placed in the recessed section 32, the outward face facing the opening of the recessed section 32.
A lead terminal, for example, the cathode lead terminal 8, bent as described above is pressed against the rolled electrodes when the unit cell 41 placed in the recessed section 32. Therefore, the cathode lead terminal 8 makes contact with the rolled electrodes and this creates a short circuit between the cathode lead terminal 8 and the rolled electrodes in some cases.
In order to prevent a short circuit from occurring therebetween, a protective tape 51 acting as an insulator is wound around the cathode lead terminal 8 fixed to the leading portion of the rolled cathode current collector 1 as shown in FIGS. 8A and 8B in such a manner that portions of the protective tape 51 overlap one another (see FIG. 4). Alternatively, an insulating sheet 52 is placed over the side face 42 as shown in FIG. 8C.
In the known battery having the above configuration, the cathode lead terminal 8 must be bent into an L shape and portions of the protective tape 51 overlap one another. This causes an increase in the thickness of the unit cell 41. Therefore, there is a problem in that the battery has a small capacity because the number of windings of the rolled electrodes must be reduced when the battery has a limited thickness. Furthermore, when the insulating sheet 52 is used, a space occupied by the insulating sheet 52 is necessary in addition to a space-occupied by the cathode lead terminal 8 bent into an L shape. Therefore, there is a problem in that the battery has a small capacity because the width of the rolled electrodes must be reduced when the battery has a limited length or width.